Exhaust gas aftertreatment systems

ABSTRACT

A method are presented for monitoring an amount of ammonia stored in a urea-based SCR catalyst based on a response of a sensor coupled downstream of the catalyst to a periodic desorbtion of a small amount of ammonia. The sensor could be an ammonia sensor, or a NOx sensor whose signal is sensitive to presence of both NOx and ammonia. The method can be performed at engine start to establish initial ammonia storage amount, or, alternatively, to adjust ammonia storage amounts or diagnose degradation when NOx conversion efficiency of the catalyst is below a predetermined value.

FIELD OF INVENTION

The present invention relates to an emission control system for diesel and other lean-burn vehicles, and more specifically, to achieving optimal NOx conversion efficiency while minimizing ammonia slip in a urea-based Selective Catalytic Reduction (SCR) catalyst by maintaining an optimum amount of ammonia stored in the catalyst.

BACKGROUND AND SUMMARY OF THE INVENTION

Current emission control regulations necessitate the use of catalysts in the exhaust systems of automotive vehicles in order to convert carbon monoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx) produced during engine operation into unregulated exhaust gasses. Vehicles equipped with diesel or other lean burn engines offer the benefit of increased fuel economy, however, catalytic reduction of NOx emissions via conventional means in such systems is difficult due to the high content of oxygen in the exhaust gas.

In this regard, Selective Catalytic Reduction (SCR) catalysts, in which NOx is continuously removed through active injection of a reductant into the exhaust gas mixture entering the catalyst, are known to achieve high NOx conversion efficiency. Urea-based SCR catalysts use gaseous ammonia as the active NOx reducing agent. Typically, an aqueous solution of urea is carried on board of a vehicle, and an injection system is used to supply it into the exhaust gas stream entering the SCR catalyst where it decomposes into gaseous ammonia (NH₃) and is stored in the catalyst. The stored ammonia reacts with the NOx in the engine exhaust gas. In such systems, urea injection levels have to be very precisely monitored and controlled. Under-injection of urea may result in sub-optimal NOx conversion, while over-injection may cause tailpipe ammonia slip. Therefore, it is important to have accurate information on the amount of ammonia stored in the catalyst, as well as to maintain an optimal amount of ammonia stored.

A typical prior art system uses multiple complicated calibration maps to estimate ammonia storage amounts in the catalyst. However, such systems are inherently inaccurate and require a large amount of system memory. Alternatively, a downstream ammonia sensor may be used to monitor slip. However, a disadvantage of such system is that ammonia sensor technology is not mature yet. Further, once slip is detected, only a corrective action of reducing reductant injection can be taken, which does not guarantee expedient elimination of slip.

The inventors of the present invention have recognized that while NOx conversion efficiency of an SCR catalyst is improved in the presence of adsorbed ammonia, it is not necessary that all of the catalyst storage capacity be utilized by ammonia in order to achieve optimal NOx conversion efficiency. Additionally, they have recognized that a NOx sensor coupled downstream of the SCR catalyst is sensitive to both NOx and ammonia present in the exhaust gas. Therefore by periodically desorbing a small portion of ammonia from the catalyst, and monitoring the corresponding increase in the NOx sensor signal, it is possible to determine the total amount of ammonia stored in the catalyst. This method can be performed periodically to prevent ammonia slip due to excessive reductant injection, via optimal reductant injection during vehicle drive cycle.

In accordance with the present invention, a method for estimating an amount of reductant stored in an exhaust gas aftertreatment device coupled downstream of an internal combustion engine includes: adjusting an operating condition to desorb a portion of reductant stored in the device; and adjusting an amount of reductant injected into the device based on a response of the sensor to said desorbed reductant portion.

In one embodiment of the present invention, the method is used to estimate an amount of ammonia stored in an SCR catalyst. The operating condition is a catalyst temperature. The sensor responding to desorbed ammonia is a NOx sensor.

In yet another embodiment of the present invention, the aftertreatment system for an internal combustion engine exhaust gas, includes: an SCR catalyst; a NOx sensor coupled downstream of said SCR catalyst; and a controller adjusting an operating condition to desorb a portion of reductant from said SCR catalyst when NOx conversion efficiency of said SCR catalyst is below a predetermined value, said controller monitoring a NOx sensor signal to determine an amount of reductant desorbed, and controlling reductant injection into said SCR catalyst based on said monitored NOx sensor signal.

An advantage of the present invention is that ammonia slip can be minimized by performing ammonia storage estimates under known operating conditions by releasing only an insignificant amount of ammonia. Another advantage of the present invention is that an accurate estimate of the amount of ammonia stored in the SCR catalyst is obtained. Therefore, improved NOx conversion efficiency can be achieved by maintaining an optimum ammonia storage amounts in the SCR catalyst. Yet another advantage of the present invention is that cost savings can be achieved by eliminating the need for an ammonia sensor downstream of the SCR catalyst. Additionally, since the downstream NOx sensor is cross sensitive to ammonia, minimizing the ammonia slip improves the accuracy of the NOx sensor readings, and thus improves overall system NOx conversion efficiency.

The above advantages and other advantages, and features of the present invention will be readily apparent from the following detailed description of the preferred embodiments when taken in connection with the accompanying drawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and advantages described herein will be more fully understood by reading an example of an embodiment in which the invention is used to advantage, referred to herein as the Description of Preferred Embodiment, with reference to the drawings, wherein:

FIGS. 1A and 1B are schematic diagrams of an engine wherein the invention is used to advantage;

FIG. 2 is a schematic diagram of an emission control system, wherein the invention is used to advantage; and

FIG. 3 is a high level flowchart of an exemplary routine for controlling the emission control system in accordance with the present invention.

DESCRIPTION OF PREFERRED EMBODIMEN(S)

Internal combustion engine 10, comprising a plurality of cylinders, one cylinder of which is shown in FIG. 1, is controlled by electronic engine controller 12. Engine 10 includes combustion chamber 30 and cylinder walls 32 with piston 36 positioned therein and connected to crankshaft 40. Combustion chamber 30 is shown communicating with intake manifold 44 and exhaust manifold 48 via respective intake valve 52 and exhaust valve 54. Intake manifold 44 is also shown having fuel injector 80 coupled thereto for delivering liquid fuel in proportion to the pulse width of signal FPW from controller 12. Both fuel quantity, controlled by signal FPW, and injection timing are adjustable. Fuel is delivered to fuel injector 80 by a fuel system (not shown) including a fuel tank, fuel pump, and fuel rail (not shown).

Controller 12 is shown in FIG. 1 as a conventional microcomputer including: microprocessor unit 102, input/output ports 104, read-only memory 106, random access memory 108, and a conventional data bus. Controller 12 is shown receiving various signals from sensors coupled to engine 10, in addition to those signals previously discussed, including: engine coolant temperature (ECT) from temperature sensor 112 coupled to cooling sleeve 114; a measurement of manifold pressure (MAP) from pressure sensor 116 coupled to intake manifold 44; a measurement (AT) of manifold temperature from temperature sensor 117; an engine speed signal (RPM) from engine speed sensor 118 coupled to crankshaft 40.

An emission control system 20, coupled to an exhaust manifold 48, is described in detail in FIG. 2 below.

Referring now to FIG. 1B, an alternative embodiment is shown where engine 10 is a direct injection engine with injector 80 located to inject fuel directly into cylinder 30.

Referring now to FIG. 2, an example of an emission control system in accordance with the present invention is described. Emission control system 20 is coupled downstream of an internal combustion engine 10 described with particular reference in FIG. 1.

Catalyst 14 is a NOx-reducing catalyst wherein NOx is continuously removed through active injection of a reductant into the exhaust gas mixture entering the catalyst. In a preferred embodiment, catalyst 14 is a urea based Selective Catalytic Reduction (SCR) catalyst in which NOx is reduced through active injection of an aqueous urea solution or other nitrogen-based reductant into the exhaust gas entering the device. The urea solution is converted into gaseous ammonia (NH₃) and stored in the SCR catalyst, wherein NH₃ serves an active NOx reducing agent. In a preferred embodiment, the SCR catalyst is a base metal/zeolite formulation with optimum NOx conversion performance in the temperature range of 200-500° C.

Oxidation catalyst 13 is coupled upstream of the SCR catalyst and may be a precious metal catalyst, preferably one containing Platinum for high conversion of hydrocarbons and carbon monoxide. The oxidation catalyst exothermically combusts hydrocarbons (HC) in the incoming exhaust gas from the engine thus supplying heat to rapidly warm up the SCR catalyst 14. The temperature of the SCR catalyst may be raised by retarding injection timing, increasing EGR and intake throttling, or any other means known to those skilled in the art to increase the temperature of the exhaust gas. Alternatively, in a direct injection engine, extra hydrocarbons may be delivered to the oxidation catalyst for the SCR catalyst warm-up by in-cylinder injection during either or both of a power or exhaust stroke of the engine. In another alternative embodiment (not shown), a reductant injection system may be used to increase the hydrocarbon concentration in the exhaust gas mixture entering the oxidation catalyst.

Particulate filter 15 is coupled downstream of the SCR catalyst for storing particulate matter, such as soot.

A reductant delivery system 16 is coupled to the exhaust gas manifold between the oxidation catalyst and the SCR catalyst. System 16 may be any reductant delivery system known to those. skilled in the art. In a preferred embodiment, system 16 is that described in co-pending U.S. patent application Ser. No. 10/301296, filed Nov. 21, 2002, assigned to the same assignee as the present invention, the subject matter thereof being incorporated herein by reference. In such system, air and reductant are injected into the reductant delivery system, where they are vaporized by the heated element and the resulting vapor is introduced into the exhaust gas mixture entering the SCR catalyst.

A pair of NOx sensors 17, 18 is provided upstream and downstream of the SCR catalyst, respectively. Measurements of the concentration of NOx in the exhaust gas mixture upstream (C_(NOx) _(—) _(in)) and downstream (C_(NOx) _(—) _(out)) of the SCR catalyst 14 provided by the NOx sensors are fed to controller 12. Controller 12 calculates NOx conversion efficiency of the catalyst, NOx_(eff). In a preferred embodiment, since a typical NOx sensor is cross-sensitive to ammonia, sensor 17 is coupled upstream of the reductant delivery system 16. Alternatively, NOx sensor 17 may be eliminated and the amount of NOx in the exhaust gas mixture entering the SCR catalyst may be estimated based on engine speed, load, exhaust gas temperature or any other parameter known to those skilled in the art to affect engine NOx production.

Temperature measurements upstream (T_(u)) and downstream (T_(d)) of the SCR catalyst are provided by temperature sensors (not shown). Controller 12 calculates catalyst temperature, T_(cat), based on the information provided by these sensors. Alternatively, any other means known to those skilled in the art to determine catalyst temperature, such as placing a temperature sensor mid-bed of the catalyst, or estimating catalyst temperature based on engine operating conditions, can be employed.

Referring now to FIG. 3, a method for controlling an amount of ammonia stored in an SCR catalyst in accordance with the present invention is presented. In a preferred embodiment, the routine in FIG. 3 is initiated when NOx conversion efficiency of the catalyst, NOx_(eff), falls below a predetermined threshold value, NOx_(eff) _(—) _(thr), such as, for example, 95%. In an alternative embodiment (not shown), the routine in FIG. 3 may be performed at engine start-up to establish initial ammonia coverage in the catalyst. In a preferred embodiment, ammonia coverage is represented by coverage fraction, θ, which is defined as a ratio of adsorbed ammonia quantity (e.g., moles) to the total storage capacity, M_(xx), wherein xx is a given set of operating conditions that influence the total storage capacity of a catalyst. If the initial ammonia coverage fraction is determined to be insufficient, reductant injection may be initiated to achieve optimal conditions.

Proceeding with FIG. 3, in step 100, a determination is made whether NOx_(eff) is less than NOx_(eff) _(—) _(thr). If the answer to step 100 is NO, the routine exits. If the answer to step 100 is YES, the routine proceeds to step 200 wherein the temperature of the catalyst is increased to release a portion of reductant stored. Various methods for increasing the temperature of the catalyst are discussed in detail in FIG. 2 above. The temperature increase, ΔT, is selected such that sufficient amount of ammonia is desorbed for a reliable determination of ammonia storage amounts, and may be defined by the following equation: $\begin{matrix} \begin{matrix} {{\Delta\quad T} \approx \frac{{\theta_{t\quad 0}^{\exp}M_{t\quad 0}^{100}} - M_{tf}^{100}}{{\mathbb{d}M_{cap}}/{\mathbb{d}T}}} \\ {\approx \frac{{\theta_{t\quad 0}^{\exp}M_{t\quad 0}^{100}} - M_{tf}^{100}}{\alpha_{2} + {\alpha_{3}C_{{NH}_{3}}^{mes}}}} \end{matrix} & (1) \end{matrix}$ where: t0=defines the time at the start of the temperature ramp for which the coverage fraction, θ, is to be predicted. tf=is the time at the end of the temperature ramp. ΔT=defines the temperature increase desired. M_(xx) ¹⁰⁰=100% storage capacity at the condition defined for the time instance of interest, xx. M_(cap)=represents the model for the storage capacity as a function of operating parameters. M _(cap)=α₀+α₁ C _(NH3) ^(mes)+α₂ T+α ₃ C _(NH3) ^(mes) *T θ_(t0) ^(exp)=expected coverage fraction at the start of the temperature ramp (based on a predetermined map of conversion efficiency as a function of catalyst coverage fraction). C_(NH3) ^(mes)=ammonia concentration as determined from the NOx sensor measurement according to the following equation: C _(NH3) ^(mes)=(C _(NOx) ^(sens)−η_(NOx) ^(exp) ×C _(NOx) ^(in))×(ω_(NH3))  (2) C^(sens) _(NOx)=NO_(x) sensor sensed value. C^(in) _(NOx)=concentration of NO_(x) at catalyst inlet. ηNO_(x)=expected NO_(x) conversion efficiency of the SCR catalyst. ω_(NH3)=1/sensitivity factor of NO_(x) sensor sensitivity to α₁, α₂, α₃=storage capacity model parameters. Next, in step 300, cumulative mass of reductant released due to temperature increase, m_(rel), is determined according to the following equation: $\begin{matrix} {m_{rel} = {\int_{t\quad 0}^{tf}{w_{exh} \times \left( {\frac{M\quad W_{{NH}\quad 3^{\prime}}}{M\quad W_{exh}}\frac{1}{10^{6}}} \right) \times \omega_{{NH}\quad 3} \times \left( {C_{NOx}^{sens} - {\left( {1 - {2\eta_{NOx}^{\exp}}} \right) \times C_{NOx}^{in}}} \right){\mathbb{d}t}}}} & (3) \end{matrix}$ where: w_(exh)=mass flow rate through the catalyst. MW_(xx)=molecular weight of the species xx. C^(sens) _(NOx)=NO_(x) sensor sensed value. C^(in) _(NOx)=concentration of NO_(x) at catalyst inlet. ω_(NH3)=1/sensitivity factor of NO_(x) sensor sensitivity to NH₃. η^(exp) _(NOx)=expected NO_(x) conversion efficiency of the SCR catalyst.

The routine then proceeds to step 400 wherein θ_(t0), the calculated coverage fraction at the start of the temperature ramp, is determined according to the following equation: $\begin{matrix} {\theta_{t\quad 0} = \frac{m_{rel} + M_{tf}^{100}}{M_{t\quad 0}^{100}}} & (4) \end{matrix}$ Next, in step 500, a determination is made whether θ_(t0) is less than θ_(t0) ^(exp), the expected surface coverage fraction, at the start of the temperature increase, by greater than some predetermined threshold constant, C. If the answer to step 500 is YES, the routine proceeds to step 600 wherein reductant injection is increased to achieve desired surface coverage fraction. In a preferred embodiment, this can be accomplished by establishing the absolute make up ammonia quantity as the sum of the amount released during temperature perturbation and the amount that needs to be added (or removed) to make up for the deficit (or excess) coverage at start of temperature perturbation: m _(NH3) ^(make) ^(—) ^(up) =m _(NH3) ^(rel) +M _(to) ¹⁰⁰×(θ^(des)−θ^(to))=(M _(to) ¹⁰⁰θ^(des) −M _(tf) ¹⁰⁰) An exemplary scheme includes a simple PI controller with the error defined as the difference between the desired makeup quantity and the integral of the output of a PI controller: ε=m_(NH3) ^(make) ^(—) ^(up) −∫{dot over (m)} _(NH3) ^(fbk) dt The PI controller maybe defined as: {dot over (m)} _(NH3,(k)) ^(fbk) =k _(prop)*(m _(NH3) ^(make) ^(—) −∫{dot over (m)} _(NH3,(k)) ^(fbk) dt)+k _(int)*(m _(NH3) ^(make) ^(—) ^(up) −∫{dot over (m)} _(NH3,(k)) ^(fbk) dt) where the subscript, k defines the current time sample, k_(prop) and k_(int) are calibratable coefficients that define the feedback dynamics such as rate, overshoot etc. The feedback quantity reduces to zero once the error reduces to zero.

If the answer to step 500 is NO, implying that the coverage fraction maybe adequately close to (or greater than) the desired coverage fraction but the conversion efficiency remains low, the routine proceeds to step 700 wherein the map for the conversion efficiency as a function of the coverage fraction and catalyst temperature is adapted to increase the desired coverage fractions for the desired conversion efficiencies. The routine then exits.

If the answer to step 500 is YES, indicating that the desired coverage fraction is below the desired value, the routine proceeds to step 600 wherein reductant injection is increased. The routine then exits.

This concludes the description of the invention. The reading of it by those skilled in the art would bring to mind many alterations and modifications without departing from the spirit and the scope of the invention. Accordingly, it is intended that the scope of the invention be defined by the following claims: 

1. A method for estimating an amount of reductant stored in an exhaust gas aftertreatment device coupled downstream of an internal combustion engine, the device having a sensor coupled downstream of it, the method comprising: adjusting an operating condition to desorb a portion of reductant stored in the device; and adjusting an amount of reductant injected into the device based on a response of the sensor to said desorbed reductant portion.
 2. The method as set forth in claim 1 wherein the exhaust gas aftertreatment device is a selective catalytic reduction (SCR) catalyst.
 3. The method as set forth in claim 2 wherein said reductant is ammonia.
 4. The method as set forth in claim 3 wherein the engine is a diesel engine.
 5. The method as set forth in claim 4 wherein said operating condition is a temperature of said SCR catalyst.
 6. The method as set forth in claim 5 wherein the sensor is a NOx sensor.
 7. The method as set forth in claim 5 wherein the sensor is an ammonia sensor.
 8. The method as set forth in claim 1 wherein said operating condition adjustment is done at engine start.
 9. The method as set forth in claim 1 wherein said operating condition adjustment is done when NOx conversion efficiency of the device is below a predetermined value.
 10. The method as set forth in claim 9 further comprising providing an indication of degradation if NOx conversion efficiency of the device remains below said predetermined value for a predetermined amount of time following said adjusting of reductant injection into the catalyst.
 11. An aftertreatment system for an internal combustion engine exhaust gas, comprising: an SCR catalyst; a NOx sensor coupled downstream of said SCR catalyst; and a controller adjusting an operating condition to desorb a portion of reductant from said SCR catalyst when NOx conversion efficiency of said SCR catalyst is below a predetermined value, said controller monitoring a NOx sensor signal to determine an amount of reductant desorbed, and controlling reductant injection into said SCR catalyst based on said monitored NOx sensor signal.
 12. The system as set forth in claim 11 wherein said operating condition is a temperature of said SCR catalyst.
 13. The system as set forth in claim 12 wherein said reductant is ammonia.
 14. The system as set forth in claim 13 wherein said controller further provides an indication of degradation if NOx conversion efficiency of said SCR catalyst remains below said predetermined value for a predetermined amount of time following said adjusting of reductant injection into the catalyst.
 15. The system as set forth in claim 14 further comprising regenerating said SCR catalyst in response to said indication of degradation.
 16. A method for controlling reductant injection into an SCR catalyst, comprising: increasing a temperature of the catalyst to a predetermined temperature; calculating an expected amount of reductant released from the catalyst due to said temperature increase based on a model of the catalyst; calculating an actual amount of reductant released from the catalyst due to said temperature increase based on a response of a NOx sensor coupled downstream of the SCR catalyst; and adjusting an amount of reductant injection into the catalyst based on a difference between said actual and said estimated amounts of released reductant.
 17. A method for controlling a NOx-reducing catalyst coupled downstream of an internal combustion engine, the catalyst having a sensor coupled downstream of it, the method comprising: providing an indication of an operating condition; in response to said indication, desorbing a portion of reductant stored in the catalyst; adjusting an amount of reductant injection into the catalyst based on a response of the sensor to said desorbtion.
 18. The method as set forth in claim 17 wherein said NOx-reducing catalyst is an SCR catalyst.
 19. The method as set forth in claim 18 wherein said sensor is a NOx sensor.
 20. The method as set forth in claim 19 wherein said sensor is an ammonia sensor.
 21. The method as set forth in claim 18 wherein said operating condition is engine start.
 22. The method as set forth in claim 18 wherein said operating condition is a condition wherein NOx conversion efficiency of the catalyst is below a predetermined value.
 23. The method as set forth in claim 22 wherein said predetermined value is not greater than 95%.
 24. The method as set forth in claim 22 further comprising regenerating the catalyst if said NOx conversion efficiency remains below said predetermined value for a predetermined amount of time following said adjustment of said amount of injected reductant. 